1. Field of the Invention
The present invention relates to a method for detecting gas leaks. More particularly, the present invention is directed to an electronic circuit that can detect natural gas at any level.
2. Description of the Prior Art
In the natural gas distribution industry, there is a need to detect and test for natural gas at several concentrations. Natural gas detection at part per million concentrations is required to locate leaks in piping and around equipment connections. Higher levels of natural gas can lead to explosive reaction. The concentration of natural gas can build to such levels over time if a leak occurs in an enclosed area. For this reason, it is necessary to measure natural gas in enclosed areas where leakage can occur and concentrations may build to levels capable of explosive reaction in order to provide warning of an explosive hazard. Natural gas concentrations beyond the explosive range can be caused by leakage into small closed volumes, such as around a buried gas pipe or in regulator pits. Natural gas concentration readings in these instances may be used to precisely locate leaks, thereby reducing the cost to repair lines by limiting the excavation necessary to expose the leaking section.
The preferred natural gas detection method can vary depending on the concentration of natural gas anticipated. For example, if part per million or explosive concentrations of natural gas are anticipated, a catalytic detector can be used. For natural gas concentrations beyond the explosive range, a detector based on thermal conductivity methods of measurement may be used.
Examples of catalytic gas detectors are described in U.S. Pat. No. 3,678,489 to Scherban et al.; U.S. Pat. No. 4,164,699 to Timoshenko et al.; U.S. Pat. No. 4,375,353 to Nicholas et al.; and U.S. Pat. No. 5,563,578 to Isenstein. U.S. Pat. No. 5,586,050 to Makel et al. discloses the use of a catalytic gas detector from Det-Tronics, Minneapolis, Minnesota. Examples of Det-Tronics detectors are Det-Tronics Model 226530-01 and CGS Detector.
Catalytic detectors typically react to a combustible gas which may come from a variety of sources with oxygen from the surrounding air. The reaction between combustible vapors and air is promoted by the catalytic material on the surface of one of two elements. The elements are electrically and thermally matched, temperature sensitive, resistive elements. The combustion reaction on the catalytically-coated element releases heat, raising the temperature of that element and, therefore, its resistance, with respect to the matching element which is not catalytically coated. This resistive imbalance is measured in a Wheatstone bridge circuit with the signal output derived being connected to additional electrical components to perform a variety of functions. U.S. Pat. No. 5,055,269 to Palumbo et al. discloses this type of detector.
Another type of gas leak detector used to detect high concentration gas leaks is based on an acoustic detection system where microphones are used to xe2x80x9clistenxe2x80x9d for the sound of gas leaking from a pipe, fitting, or valve. Such acoustic methods are disclosed in U.S. Pat. No. 5,117,676 to Chang; U.S. Pat. No. 5,416,724 to Savic; and U.S. Pat. No. 5,650,943 to Powell et al. These methods have a drawback in that in order for a leak to be audible, it must be significant. Therefore, early detection is not provided.
An expensive method developed for natural gas detection is based on the use of infrared absorption. U.S. Pat. No. 4,507,558 to Bonne and U.S. Pat. No. 4,958,076 to Bonne et al. disclose methods of selective natural gas detection utilizing absorption of infrared radiation. These methods are limited primarily to low concentration detection.
Still another method for detecting natural gas is described in U.S. Pat. No. 4,013,943 to Chou et al. This method involves a solid state electrolytic cell sensor that causes the dissociation of the gas into charged species, such as ions and complex ions. This method also utilizes a heater, which can be dangerous and is limited as to how low the detection limit for natural gas can be.
There remains a need for a safe and inexpensive means for detecting natural gas at any concentration.
The present invention is directed to a unique circuit capable of detecting natural gas at a wide range of concentrations. The gas detection circuit includes a catalytic Wheatstone bridge circuit and an analyzing Wheatstone bridge circuit. The catalytic Wheatstone bridge circuit includes an active element and a reference element on a first half of the catalytic Wheatstone bridge circuit and a first fixed resistor and a second fixed resistor on a second half of the catalytic Wheatstone bridge circuit. The analyzing Wheatstone bridge circuit includes the active element and the reference element in series, acting as a single element in series with a third fixed resistor on a first half of the analyzing Wheatstone bridge circuit. A second half of the analyzing Wheatstone bridge circuit includes a fourth fixed resistor and a fifth fixed resistor, which balance the first half of the analyzing Wheatstone bridge circuit.
The catalytic Wheatstone bridge circuit and analyzing Wheatstone bridge circuit are powered by maintaining a fixed voltage difference across the respective Wheatstone bridge circuits. An output signal is taken from the midpoint of the half bridge at the juncture of the active element and the reference element for comparison to the voltage at the midpoint of the two matched balancing resistors, which are the first fixed resistor and the second fixed resistor. The intensity of the signal is proportional to the concentration of natural gas.
The present invention is also directed to a method of detecting natural gas. The method includes placing a device containing the circuit described above in an area in which the air has a potential of containing natural gas and monitoring the output signal for indications of the presence of natural gas.
These and other advantages of the present invention will be clarified in the description of the preferred embodiment taken together with the attached drawings in which like reference numerals represent like elements throughout.